174 



THE SECOND-CHROMOSOME GROUP 



back-cross — that the first back-cross was a test of the amount of 

 crossing-over in the male and the second was of crossing-over in 

 females. Up to this time there had been no suspicion that the result 

 of a back-cross could be in any way dependent on the sex of the Fi 

 parent used in the experiment. From this evidence it was concluded 

 that there was crossing-over in the male, but that it was of different 

 degree from that in the female. In September 1912, Morgan showed 

 that in the case of black vestigial no crossing-over whatever had 



Table 29. — B. C. offspring given by Fi daughters, from the out-cross of a 

 purple vestigial male to a wild female, when hack-crossed to purple vestigial 

 males. 



occurred in the male, while in the female there was even more crossing- 

 over than had been found in the case of purple- vestigial. Subsequent 

 tests, including hundreds of thousands of individuals, have shown that 

 ordinarily there is no crossing-over in the male for any chromosome 

 and that the few (2) cases that have occurred were probably not 

 brought about by the same mechanism as that by which crossing-over 

 is ordinarily effected. 



NO CROSSING-OVER IN THE MALE. 



A clear conception of the fact of no crossing-over in the male was 

 obscured in the original vermiUon purple vestigial back-cross test by 

 the apparent occurrence of cross-overs in one of the five cultures. It 

 is still a matter of doubt as to what actually occurred in that culture. 

 No tests were made of the apparent cross-overs, because there was at 

 that time no evidence, aside from the inconsistency within the experi- 

 ment, to suggest that they were very unusual. It is strongly sug- 

 gestive of error that in this first male test, carried out before we 

 were on guard, so many apparent cross-overs should have occurred, 

 and that in the numerous and extensive tests made subsequently they 

 should be so strikingly absent. Perhaps some clerical error was 

 committed — such, for example, as a mistake in labeling — that this 

 culture was really one of the F2 cultures that had been made up from 

 the same Fi culture, though at a different time from the back-crosses. 



